Conventionally, in an electroacoustic transducer called “Gamuzon type speaker,” such a type has been used, in which an acoustic diaphragm on which a flat coil pattern of an electric conductor corresponding to a voice coil is formed is installed at a pair of intermediate parts of a magnetic field generator, and a drive current is supplied to the electric conductor, wherein the acoustic diaphragm is vibrated in the perpendicular direction to the plane thereof.
The acoustic diaphragm of the Gamuzon type speaker is structured so that an electric conductor is disposed almost on the entire surface of the acoustic diaphragm, and is featured in that the entire surface is driven at the same phase and favorable transient characteristics may be obtained at a wide range.
For example, (Patent Document 1) discloses an electroacoustic transducer, in which adjacent band-shaped magnets (or band-shaped areas in a magnet plate) are disposed with the N and S poles thereof made different alternately, the entirety of a magnet plate composed of a number of band-shaped magnets is formed to be like a flat plate, the N and S poles are disposed with the directions thereof made perpendicular to the flat plate surface, and an acoustic diaphragm, in which electric conductors are formed, is disposed opposite to the flat surface of the magnet plate.
In the electroacoustic transducer, since the N and S poles are disposed with the directions thereof made different alternately, there are many parts where the directions of the magnetic field are inverted on the acoustic diaphragm and many parts where the magnetic flux density is low. Therefore, the density of magnetic flux to drive the acoustic diaphragm in the direction perpendicular to the plane, that is, the density (hereinafter called a “magnetic flux density for effective action”) of magnetic flux (hereinafter called a “magnetic flux for effective action”) by which the electromagnetic force operating on the electric conductors of the acoustic diaphragm is turned into the vibration direction was subjected to a large change. Also, it was necessary that the winding direction of the electric conductor is inverted in accordance with the direction of inverting magnetic fields and that the electric conductors are arranged in accordance with a partially existing area where the magnetic flux density for effective action is high. Therefore, it is not possible to form the entire surface of the diaphragm of a planar coil, and a supporting member such as a synthetic resin sheet to support the planar coil is indispensable. Sound inherent to the supporting member adversely influences the sound quality. Further, there is another problem that great unevenness is brought about in the drive force of respective parts of the diaphragm, which becomes a factor by which separate vibrations become a critical problem for reproduction of high quality sound.
Further, (Patent Document 2) discloses an electroacoustic transducer in which two magnet plates having a columnar-shaped magnet and a ring-shaped magnet separately arranged concentrically at the center side and the outer circumference side are opposed to each other, an acoustic diaphragm (planar coil diaphragm) having electric conductors spirally printed thereon is disposed between the two magnet plates parallel to the magnet plates, and the polarities are inverted at the center side and the outer circumference portion with the magnetization direction of magnets turned into the direction perpendicular to the acoustic diaphragm.
In the electroacoustic transducer, since the electric conductors are spirally wound in the same direction, it becomes possible to form the entire surface of the diaphragm of a planar coil. Therefore, it becomes possible that a drive force may be generated on the entire surface of the diaphragm, wherein this is effective in response to such a problem as in (Patent Document 1). However, since the area at which the magnetic flux density is high is made very narrow in the arrangement of magnets the magnetization direction of which is brought into only the direction perpendicular to the diaphragm, it was impossible to widen the area of the diaphragm. Therefore, it is difficult to adopt the electroacoustic transducer as a low frequency range speaker for which the diameter of the diaphragm is increased, wherein use is restricted to a high frequency range speaker in order to use in a state where the utilization efficiency of magnetic flux is high. In addition, since fluctuations in the magnetic flux density for effective action are large at respective positions of the planar coil, it is not possible to obtain a uniform drive force on the entire surface of the diaphragm, wherein a problem of separate vibrations could not be solved.
Thus, since the area at which the magnetic flux density for effective action is high becomes very narrow in prior art magnet plates the magnetization direction of which is brought into only the direction perpendicular to the diaphragm, it is not possible to widen the area of the diaphragm. Although in (Patent Document 2) the area is widened by widening the gap between the respective magnets at the center side and the outer circumference side, which become two partial areas, as a countermeasure, the magnetic flux density for effective action is lowered in this case, wherein the width is restricted. Finally, it is difficult to adopt such prior art electroacoustic transducers as a low frequency range speaker in which the diameter of the diaphragm is increased. In addition, in almost all cases since the magnetization direction is one direction, the distribution of the magnetic flux density for effective action is adjusted by varying the gap between two types of partial areas. With such restricted adjustment, it is difficult to make uniform the distribution of the magnetic flux density for effective action at respective positions of the acoustic diaphragm, and it is difficult to obtain a uniform drive force at the entire surface of the diaphragm.
In order to solve these problems in the prior arts, (Patent Document 3) that the present applicant earnestly researched and developed and to which a patent right was granted discloses an electroacoustic transducer in which, using a magnet plate (hereinafter called a “magnet plate of optimum magnetization angle”) that is divided into a number of partial areas and the respective partial areas are turned into magnetization directions in order to increase the use efficiency of a magnet, an acoustic diaphragm in which an electric conductor is spirally wound is disposed parallel to the magnet plate at the front of the magnet plate. Also, the patent document further discloses an electroacoustic transducer having a similar acoustic diaphragm installed therein, using a magnet plate (hereinafter called a “magnet plate magnetized in the radius direction”) in which a component parallel to the vibration plane of the acoustic diaphragm is turned into a radius direction of the magnet plate in the magnetization direction of the magnet plate, and in which the angles formed with respect to the vibration plane of the acoustic diaphragm are all fixed.    [Patent Document 1] Japanese Published Examined Patent Application No. S35-10420    [Patent Document 2] Japanese Published Unexamined Utility Model Application No. S60-93397    [Patent Document 3] Japanese Patent No. 3612319